This application proposes two related studies: A population-based epidemiologic study of obesity and depression among women and a longitudinal study of obesity treatment among two cohorts (one with comorbid obesity and depression, one with obesity only) identified by the epidemiologic study. Study 1 - Epidemiologic Study: A population-based sample of approximately 6000 women aged 40-65 will be complete structured telephone assessment of weight, nutrient intake, physical activity, depression, functional impairment, and disability. Women with Body Mass Index (BMI) >30 will be oversampled. Insurance claims data will be used to measure health care costs. Aims of the epidemiologic study include: 1) Examine the association between obesity and depression among middle-aged women 2) Examine the specific contributions of obesity and depression to disability and health care costs. Study 2 - Treatment study: A cohort of approximately 100 women with obesity (BMI > 30) and no current depressive disorder will be enrolled in a 6-month state-of-the-art group weight loss treatment. Approximately 200 women with comorbid obesity and depression will be randomly assigned to either the identical weight loss treatment or to a combined cognitive-behavior group therapy program focused on both depression and weight loss. Aims of the treatment study will include: 1) Examine the effect of depression on success in weight loss treatment by comparing weight loss, diet, and exercise in depressed and non-depressed women enrolled in the identical weight loss program 2) Examine the benefits of a combined weight loss/depression intervention above those of weight loss treatment alone by comparing weight loss, nutrient intake, physical activity, depressive symptoms, functional impairment and disability in the two groups of women with comorbid obesity and depression randomly assigned to the two different intervention programs